This application claims priority to an application entitled, xe2x80x9cDispersion-Managed Fiber Preform and Fabricating Method Thereof by MCVDxe2x80x9d, filed in the Korean Industrial Property Office on Jan. 14, 2000 and there duly assigned Serial No. 2000-1742.
1. Field of the Invention
The present invention relates generally to a method for fabricating an optical fiber preform, and in particular, to a method for fabricating a dispersion-controlled fiber preform by a modified chemical vapor deposition (MCVD) process.
2. Description of the Related Art
Due to the wide bandwidth capabilities of optical fibers, the demand for optical communication has increased drastically. Basically, optical fibers have a bandwidth that is wide enough to exchange a large amount of data and video signals via a strand of optical fiber. The transmission capacity of an optical fiber increases as its dispersion value at the transmission wavelength decreases. Thus, in order to reduce the dispersion in the transmission window having the lowest attenuation, an optical fiber with a dispersion value of zero at 1550 nm (referred to as dispersion shifted fiber (DSF)) has been introduced. This dispersion shifting creates a fiber that shifts the low-dispersion region to the 1550-nm region.
In addition, the demand for optical communication is fueled by the late development of an erbium-doped fiber amplifier (EDFA). The EDFA can amplify optical signals into a wide wavelength band that ranges from 1530 nm to 1565 nm, thereby rendering the WDM (Wavelength Division Multiplexing) scheme viable as well as increasing the optical transmission capacity. Normally, optical signal channels at different wavelengths are transmitted via one optical fiber in the WDM scheme. However, a non-linear reaction between signals at different wavelengths, for example four-wave-mixing, significantly deteriorates the system performance in the WDM optical communication when a dispersion shifted fiber (DSF) is used.
Andrew R. Chraplyvy addressed this performance problem by introducing an optical fiber with a dispersion in the range of 2-6 ps/km-nm in the optical transmission wavelength band, as disclosed in U.S. Pat. No. 5,327,516 entitled, xe2x80x9cOptical Fiber for Wavelength Division Multiplexing.xe2x80x9d
An optical fiber with a very low dispersion value for suppressing the non-linear effect is known as an NZDSF (Non-Zero DSF). Major providers of the NZDSFs are the TrueWave ((trademark)) of Lucent Technology and LEAF ((trademark)) (Large Effective Area Advantage Fiber) of Corning. The TrueWave ((trademark)) RS optical fibers of Lucent Technology are designed to have low dispersion values at the L-band range (Long-band 1565-1620 nm) as well as at the C-band range (Conventional-band 1530-1565 nm). On the other hand, the LEAF ((trademark)) fibers of Corning have a wider effective area when compared to other optical fibers in the market. For details, refer to U.S. Pat. No. 5,835,655, entitled xe2x80x9cLarge Effective Area Waveguide Fiberxe2x80x9d by Yanming Liu, et. al. The advantage of the NZDSFs is that they provide wide effective areas as the non-linear effect of an optical fiber decreases.
Because the dispersion value of the NZDSF is very low but not zero, the products of the dispersion value and the transmission distance continue to increase when the transmission distance is long. As a result, the dispersion inevitably results in pulse spreading. To solve this problem, the so-called dispersion management has been suggested in which two NZDSFs with a positive dispersion value and a negative dispersion value, respectively, are connected in an alternate arrangement to prevent the dispersion accumulation. Such dispersion-managed WDM system is an ideal optical transmission system. To build a dispersion-managed line, optical fibers with a positive dispersion value and a negative dispersion value are separately fabricated and then alternately connected. However, this conventional method makes it difficult to install optical communication cables because different kinds of optical fibers must be installed alternately along the long optical fiber line. What is needed is an improved technique for manufacturing dispersion-managed optical fibers from a prefrom that is prepared by one continuous session of a modified chemical vapor deposition (MCVD) process.
It is, therefore, an object of the present invention to fabricate a dispersion-managed fiber preform using a modified chemical vapor deposition (MCVD) process.
It is another object of the present invention to provide a dispersion-managed fiber preform with a positive dispersion length for a first predetermined length and a negative dispersion value for a second predetermined length.
It is a further object of the present invention to provide a dispersion-managed fiber preform from which an optical fiber is drawn to have a positive dispersion value for a first predetermined length and a negative dispersion value for a second predetermined length.
It is still another object of the present invention to provide a dispersion-managed fiber preform that is uniformly deposited lengthwise and selectively etched to have different refractive index profiles along the length direction prior to the tube sealing under the MCVD process.
It is yet another object of the present invention to provide a dispersion-managed fiber preform that has the refractive index distributions of an NZDSF+ and an NZDSFxe2x88x92, by making the core portions different in thickness and an identical refractive index distribution in the other core portion.
To achieve the above objects, a core and clad having the refractive index distribution of an optical fiber with a positive dispersion value are uniformly deposited in a glass tube. The preform with the positive dispersion value is heated at every predetermined period with a heater and then the heated preform portions are etched by fluorine to have a negative dispersion value. Thereafter, the preform with the alternate, positive dispersion value and the negative dispersion value along the length direction is collapsed.